Page 36 - Petroleum Geology

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North America (see Matsumoto, 1980). These events were of great signifi-
cance for the accumulation of petroleum, and we shall examine them in
more detail in a later chapter. We shall see that these transgressions were but
an episode in a world-wide Mesozoic tectonic event that may well have in-
volved all four causes listed above.
The fact that transgressions seem to have a world-wide tendency more
commonly than regressions is probably due to the amplifying effect of a
rising sea level on subsidence of a sedimentary basin compared with falling
sealevel on subsidence. Regional variations in subsidence rates may account
for the lack of strict contemporaneity because we observe only the net effect
of subsidence and sediment accumulation. It is clear that orogeny may
generate enough sediment to lead to a regressive sequence in spite of a con-
temporary rise of sealevel, and so mask it; but it is unlikely that subsidence
will so exceed the rate of sea-level fall that a transgressive sequence of any
magnitude will accumulate during a period of eustatic fall of sea level. For
example, the Catskill delta of the northern Appalachians developed during
the middle and late Devonian, as a result of the Acadian orogeny, while
transgression was taking place in western Canada, western Australia and
north-west Europe.
Vail et al., in a series of papers (see, for example, Vail et al., 1977), con-
cluded from their studies of the seismic stratigraphy of many areas of the
world that numerous synchronous, and so global events are recorded in
sedimentary basins around the world. These they interpret as eustatic events.
Morner (1976) believes that true eustatic changes of sea level are unlikely
because the geoid is probably unstable even on a short timescale. He pointed
out that there is a 180 m difference in geoid level over 50"-60" of longitude
between the Maldive Islands (-104 m) and New Guinea (+76 m on the
Smithsonian Standard Earth I11 geoid map that he used) (Fig. 1-5). An easterly
drift of the geoid without crustal adjustment would lead to a fall of sea level
of about 180 m around New Guinea (which would reunite it with Australia)
and a rise around the Maldives of about 80 m (which would inundate them).
Geoid relief may have been greater in the past, but the causes of this relief
are not yet well understood. There is some similarity between the geoid
surface and the non-dipole magnetic field (see Hide and Roberts, 1961, fig.
2.3b).
Petroleum exploration around the world will doubtless clarify these
problems in time as our detailed knowledge and understanding of sedimen-
tary basins is extended.

LITHOLOGICAL ASSOCIATIONS IN SEDIMENTARY BASINS

If one considers the sedimentary rocks broadly as mudstone/shales, sands/
sandstones, and carbonates/evaporites, sedimentary basins tend to accumulate